In recent years surface mounting technology has been introduced for electrically interconnecting microelectronic devices such as flip chip semiconductor devices to wiring boards. This technology permits the attainment of a higher density of interconnection as compared to conventional methods of interconnection. Conventional methods of interconnection include wire bonding and solder reflow. High density application using flip chip surface mounting attachment technology offers the advantage of miniaturization. Methods of flip chip interconnection include solder bump interconnection, conductive adhesive bonding, gang bonding for polyimide tape carrier system, and pressure engaged adhesive interconnection. The solder fellow and tape interconnection methods both possess pitch limitations. Adhesives, on the other hand, permit the attainment of fine pitch connections.
Hatada U.S. Pat. No. 4,749,120 discloses a method of connecting flip chips to a wiring board by means of a resin of a modified acrylate group in which light and heat are used in addition to pressure in order to stiffen the resin. A New LSI Bonding Technology Micron Bump Bonding Assembly Technology, presented at the 39th Electronic Components Conference, May 22, 24, 1989, Houston, 0569-5503/89/0045 IEEE and CH 25484/88/0000-0023 IEEE discloses the use of a light setting insulating resin to bond a large scale integration (LSI) chip to the electrodes on a substrate. Special Issue/Assembling Mounting Technology: Recent Trends in Wireless Bonding Technolgy by Hatada et al., Semiconductor World, September, 1987, discloses the bonding of chip electrodes to electrodes on a substrate using a light setting acrylate and epoxy insulation resin. Each of the above resins are prepared according to conventional free radical polymerization processes. Thus, the processes once started and stopped provide materials of certain physical properties and shape which cannot be further modified without the inclusion of additional initiator. Thus, the above-described polymerization processes do not possess a "living" character.
When using transparent wiring boards, ultraviolet (UV) light curable adhesives are highly desirable for flip chip attachment because it is possible to take advantage of the transparency of the wiring board when curing the adhesive. UV light curing of adhesives is a fast and clean process. However, the cure of known UV light curable adhesives is not easy to control. The polymerization of known UV light curable adhesives is a one-step reaction preventing the possibility of reaching intermediate hardening stages. In flip chip attachment, it is desirable to be able to control the modulus of the cured adhesive, and thus the electrical connection between the flip chip and the wiring board. Soft undercured adhesive materials do not possess good dimensional stability whereas hard overcured adhesive materials are characterized by high stress levels in the bonds. It is therefore, an object of this invention to provide a non-tacky thermoplastic radiation curable adhesive for the attachment of microelectronic devices, which can be controllably cured to provide exactly the amount of cure and hardening desired. We have discovered such adhesive compositions which employ iniferter technology.
The term "iniferter" refers to a chemical compound that has a combined function of being a free radical initiator, transfer agent, and terminator, the term "iniferter" being a word formed by the underlined portions of the terms identifying these functions. This term and its use in the production of block copolymers is well known, particularly because of the work of Takayuki Otsu of the Department of Applied Chemistry, Osaka City University, Osaka, Japan. This work is discussed, for example, in an article by Otsu, et al. entitled "Living Radical Polymerizations in Homogeneous Solution by Using Organic Sulfides as Photoiniferters," Polymer Bulletin, 7, 45-50 (1982), an article by Otsu, et al. entitled "Living Mono- and Biradical Polymerizations in Homogeneous System Synthesis of AB and ABA Type Block Copolymers" Polymer Bulletin, 11, 135-142 (1984), and in European Patent No. 0286376, published Oct. 12, 1988. Despite the rather detailed description of making certain block copolymers according to such disclosures, there is no disclosure of non-tacky thermoplastic radiation curable adhesive compositions which can be controllably cured in order to bond microelectronic devices to transparent wiring boards.
Copending U.S. application Ser. No. 07/356,650, filed May 19, 1989, which is a continuation-in-part of U.S. application Ser. No. 07/212,594, Ali, et al., filed Jun. 28, 1988, (assigned to the assignee of the present case) discloses the use of iniferter technology in the preparation of acrylic block copolymers having the requisite physical properties of adhesion, cohesion, stretchiness, and elasticity, making them suitable for use in pressure-sensitive adhesive compositions. Ser. No. 07/356,650does not teach non-tacky thermoplastic radiation curable adhesive compositions which can be controllably cured in order to bond microelectronic devices to transparent wiring boards.